As telecommunication systems evolve from 10 Gb/s transmission rate to 40 Gb/s and now 100 Gb/s, more sophisticated modulation scheme are developed. For example, phase modulation is increasingly used rather than simpler amplitude modulation. At 100 Gb/s, coherent communication involving polarization multiplexing further adds to the required complexity at the transmitter and receiver sides. In particular, more optical processing is required and calls for photonic integrated solutions.
The more complex hardware required at the transmitter and receiver sides also mean additional physical space. The integration on both optical and electronic devices thus implies considerable amount of space in new systems.
For at least these reasons, there is an immediate need to reduce the size of the optical functions that are implemented into optical systems. To this end, using optical integrated circuits can be an excellent approach for reducing by orders of magnitude the size of optical devices. Yet, efficient coupling of optical fiber inputs and outputs to a waveguide substrate remains a challenge when using integrated optics.
Connecting optical fibers to lightwave circuits may be achieved by various techniques. When the cross-sectional area of the fiber core is larger in size than that of the waveguide, the use of diffraction grating couplers is one of the most power efficient ways for coupling an optical signal. In a typical configuration, a diffraction grating is positioned on the surface of the waveguide and the signal enters the diffraction grating at a nearly normal angle from the surface.
The angle of incidence on the diffraction grating needs to be close to perpendicular, but a small incidence angle is preferred to avoid the strong back reflection due to the second order of diffraction of the grating. An angle of between 5° and 20° from the vertical (i.e. the normal to the surface of the waveguide) is typical, but diffraction grating designs can be made to accommodate angles of incidence of up to 30°.
Attachment of the fiber with a butt coupling at a normal angle from the waveguide surface is not very practical for integration into systems, since it requires considerable additional space. For examples, the waveguide may include opto-electronic functions (e.g. photodiodes, variable optical attenuators, lasers, and the like) whose integration is normally made in the same plane as that of the circuit board on which the integrated photonic chip is typically mounted. Coupling at a normal angle thus implies that the optical fiber would exit perpendicularly to the supporting circuit board, which would be impractical in a dense integration scheme.
One approach known in the art has been proposed in U.S. Pat. No. 7,162,124 the contents of which are incorporated herein by reference, and consists in cutting the end of the fiber at 45° or less with respect to the fiber core in order for light propagating therein to be reflected either by total internal reflection or from a coated mirror deposited on the cleaved surface defined by the cut angle. This geometry allows the fiber to be parallel to the waveguide and reduces significantly the size of the assembly.
One drawback of the angled tip reflection is that the output light diverges when exiting the core of the fiber and goes through its cladding. The diverging angle causes power losses that are detrimental in high efficiency telecom applications. In order to overcome this problem, it has been proposed in U.S. Pat. No. 7,162,124 to reduce the fiber cladding thickness while maintaining the fiber parallel to the substrate. While resolving the power efficiency problem in theory, this method proves to be very difficult to implement efficiently in production. This is mainly due to the difficulty of precisely polishing the fiber core to an exact length along a significant portion of the fiber, since standard polishing techniques and equipment cannot be used.
There therefore remains a need for an improved optical coupling between an optical fiber and a waveguide that alleviates at least some of the above-mentioned drawbacks.